1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus having a gradient system and a radio-frequency system that, among other things, are utilized for location coding of magnetic resonance signals, whereby magnetic resonance signals from at least parts of a region of an examination subject to be imaged are acquired in a time sequence.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of a subject to be examined. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system of the apparatus are superimposed on a static basic magnetic field. For triggering magnetic resonance signals, further, radio-frequency signals are emitted into the examination subject, which trigger magnetic resonance signals that are picked up, and image data sets and magnetic resonance images are produced on the basis of the picked-up signals. The magnetic resonance signals that are picked up are demodulated in phase-sensitive fashion and are converted into complex quantities by sampling and analog-to-digital conversion. The complex quantities are entered in a k-space matrix from which a magnetic resonance image can be reconstructed with a multi-dimensional Fourier transformation.
Among other things, the gradient fields are used for location coding, i.e., the gradient fields make the contributions of individual voxels of a region of the examination subject to be imaged distinguishable in the registered magnetic resonance signal. To that end, the gradient fields are used in combination with the radio-frequency signals for selective excitation of a prescribable region of the examination subject, for example a slice, and also are used for the spatial coding within an excited region, for example a slice or a larger volume.
Generating magnetic resonance images free of motion artifacts requires an identical geometrical positioning of the region to be imaged over an entire exposure time span. Particularly for in vivo imaging, physiological movements as are caused by a heart action, respiration and/or a peristaltic of organs, oppose this requirement. For eliminating artifacts as a consequence of respiratorial movements, for example, magnetic resonance signals are only excited and/or registered during a reproducible respiratory phase. It is also known to retrospectively correct magnetic resonance images registered independently of the respiratory movement according to a time curve of the respiratory movement. In both techniques, the respiratory movement must be acquired, for example, with a respiration belt according to German OS 39 35 083 with which the respiratory movement is converted into a pressure signal that is forwarded via a pressure tube to an optical pressure sensor.
In functional magnetic resonance imaging, for example, three-dimensional image data sets of the brain are registered every two through four seconds, frequently with an echo planar method. After many image data sets have been registered at various points in time, the image data sets are compared to one another for signal differences for identifying active brain areas in order to form images referred to as activation images. The slightest positional changes of the brain during an overall exposure time span of the functional magnetic resonance imaging thereby lead to undesired signal differences that mask the sought brain activation.
In one embodiment of a functional magnetic resonance imaging, a correlation referred to as prospective motion correction is implemented during an executive sequence of the functional magnetic resonance imaging. To that end, positional changes which may have occurred, i.e. rotations and translations, of the region to be imaged are acquired from image data set registration-to-image data set registration, for example, on the basis of orbital navigator echos, and the location coding is adapted during the executive sequence for compensating the acquired positional changes.
An orbital navigator echo is registered just like a magnetic resonance signal generated for imaging, and the complex-number values thereof are entered in a navigator echo matrix as data points in k-space, with the data points forming a circular k-space path. A positional change between the points in time can be detected on the basis of orbital navigator echos that are generated at different points in time. To that end, a navigator echo is registered, for example, before every registration of an image data set, and the resulting navigator echo matrix is compared to a reference navigator echo matrix for identifying a positional change.
As is known, there is a linkage between image space and k-space via the multi-dimensional Fourier transformation. A translation of the region to be imaged in image space is expressedxe2x80x94according to the shift rule of Fourier transformationxe2x80x94as a modified phase of complex-number values in the associated k-space matrix of the region to be imaged. A rotation of the region to be imaged in image space causes the same rotation of the associated k-space matrix. In order to decouple a rotation from a translation in k-space, only magnitudes of the complex-number values are considered for rotations. Compared to a reference point in time, thus, a rotation of the region to be imaged can be determined by comparing magnitude values of the navigator echo matrix to those of the reference navigator echo matrix. The phase values are compared for determining a translation. The location coding is correspondingly adapted for compensating for a positional change identified in this way for an image data set to be subsequently registered. Because the occupation of k-space matrices in magnetic resonance technique can be directly controlled by the gradient fields, translations and rotations of the region to be imaged can be directly compensatedxe2x80x94according to the rules of Fourier transformationxe2x80x94by an appropriately modified gradient field setting. The same applies for selective excitation of a prescribable region of the examination subject.
Respective orbital navigator echos are generated in three planes orthogonal relative to one another for identifying arbitrary positional changes in three-dimensional space. The article by H. A. Ward et al., xe2x80x9cProspective Multiaxial Motion Correction for fMRIxe2x80x9d, Magnetic Resonance in Medicine 43 (2000), pages 459 through 469, an example of a discussion of the above-described use of orbital navigator echos.
An object of the present invention is to provide a method for operating a magnetic resonance apparatus with which, among other things, a magnetic resonance image can be registered free of artifacts as a consequence of deformations of a region to be imaged.
This object is achieved in a method according to the invention for operating a magnetic resonance apparatus having a gradient system and a radio-frequency system that, among other things, are utilized for location coding of magnetic resonance signals, wherein magnetic resonance signals from at least parts of a region of an examination subject to be imaged are acquired in a time sequence, and wherein a deformation of the region to be imaged occurring during the time sequence is identified and the location coding is adapted according to the identified deformation.
Without retrospective correction, magnetic resonance images of a deformed region to be imaged can be registered which are free of artifacts as a consequence of the deformations.
In an embodiment, the deformation is identified with a navigator echo technique. As a result, utilization of a respiratory belt for acquiring the respiratory movement is not needed to identify a deformation that is produced by respiration of the examination subject.